For an apparatus for recording and reproducing optical information, a high density can be realized by using a light source having a shorter wavelength. For example, while near infrared light of 780 nm is used for a widespread compact disk apparatus, a red semiconductor laser of 650 nm is used for a digital versatile disk (DVD) realizing a higher density information reproduction. Furthermore, for realizing a high density next-generation optical disk apparatus, a blue laser light source having a shorter wavelength has been developed keenly. For example, a wavelength converter using a nonlinear optical material has been developed for a small and stable blue laser light source.
For example, a QPM (quasi-phase matching)-SHG (second harmonic generation) element using a periodically domain-inverted structure can realize a highly efficient wavelength conversion. However, as the tolerance of the conversion wavelength is extremely narrow, a wavelength of an excitation light source should be fixed to a phase matching wavelength in order to obtain stable light source properties. For providing a small light source, a semiconductor laser must be used for the fundamental light source. However, since such a semiconductor laser has a wide gain and the oscillation wavelength fluctuates easily, a technique for stabilizing the wavelength will be highly valued. Here, a phase matching wavelength denotes a wavelength of a fundamental wave in a case where a fundamental wave transmitted in a periodically domain-inverted structure is converted to a harmonic.
A technique for providing a stable light source by using a QPM-SHG element and a semiconductor laser is an integration of a DBR (Distributed Bragg Reflector) grating in such a QPM-SHG element. This method includes using a DBR grating that has a wavelength selectivity so that an oscillation wavelength of a semiconductor laser is fixed to a reflection wavelength of the DBR grating by means of optical feedback.
Specifically, for an optical waveguide type QPM-SHG element, a DBR grating structure is formed on the optical waveguide. Such a DBR grating can be formed on the optical waveguide by etching, resist-grating or the like. In such a configuration, by feeding back a reflected light beam from the DBR grating to the semiconductor laser, an oscillation wavelength of the semiconductor laser can be fixed to this reflection wavelength.
For a method of forming a DBR grating on a SHG element, the use of a periodically domain-inverted structure for quasi-phase matching, as a DBR grating, has been proposed. For the periodically domain-inverted structure, a Ti diffusion LiNbO3 is used. It is known that when Ti is diffused thermally into LiNbO3, the polarization at the diffusion part will be inverted. Since the refractive index at this diffusion part is increased at the same time, it functions also as a refractive index grating having a periodic change in refractive index. Therefore, the domain-inverted structure can be used for the DBR grating. And the oscillation wavelength of the semiconductor laser can be fixed by harmonizing the DBR reflection wavelength and the phase matching wavelength. Thereby, a wavelength converter with a stable output can be realized. That is, a short wavelength light source having a semiconductor laser and a SHG element integrated with each other will be provided.
A similar method is applied for a KTP crystal. When a KTP crystal is subjected to an ion-exchange with Rb, the polarization is inverted and at the same time the refractive index at the ion-exchange portion is increased. This phenomenon is used to form a domain-inverted structure, and thereby a QPM structure and a DBR grating can be formed. In this manner, a short wavelength light source is provided.
Methods for forming such a DBR grating are disclosed, for example, in JP H06(1994)-194708.
However, the above-mentioned domain-inverted structure provides a change in refractive index as well as the domain-inverted structure, since it is accompanied with a composition change of the crystal. The reason is that metal diffusion or ion-exchange has been used in forming a domain-inverted structure. However, such a change in crystal composition can result in degradation of the crystal itself, and it may cause problems such as degradation of the crystal and reduction of resistance to optical damage. Thus such a structure cannot be used for an optical waveguide device that enables transmission of high-output light at high efficiency.
For example, a periodically domain-inverted structure formed by Ti diffusion on a LiNbO3 crystal can be used as a DBR grating, as it is accompanied with an increase in refractive index. However, the domain inversion caused by the Ti diffusion on the LiNbO3 substrate will not provide a sufficient depth of the domain inversion. Particularly, it is difficult to form a domain-inverted structure having a period of not more than 3 μm, which is required for a wavelength of about 400 nm or less. Moreover, when Ti is diffused, impurity concentration in the optical waveguide increases and the resistance to optical damage will deteriorate. As a result, the output may be unstable at the time of output that is as high as at least several milliwatts.
In a method of ion-exchanging the KTP crystal with Rb, a domain-inverted structure and a change in refractive index can be provided by the Rb ion exchange. However, since the Rb as an impurity enters the crystal, it will be difficult to provide high output, similarly to the case of Ti diffusion. Furthermore, since a large difference in the grating constants is provided between the ion exchange portion and non-exchange portion due to the ion exchange with the Rb, distortion or the like may occur in the crystal at the inverted part, thereby increasing a loss in the transmission in the optical waveguide.
Recently, techniques for domain inversion in a nonlinear optical crystal have been improved further. The above-mentioned methods of forming domain inversion by means of an initial ion exchange or metal diffusion have been replaced by a method of producing a domain-inverted structure through application of a high electric-field pulse by means of a pattern electrode. Specifically, a method of applying a high voltage pulse to a ferroelectric substance via a pattern electrode enables formation of a uniform domain-inverted structure without changing the composition of the crystal, and further formation of a domain-inverted shape with a high aspect ratio. When forming a domain-inverted structure in this method, the direction of the nonlinear polarization will be inverted by changing slightly a distribution of atoms in the crystal, and this will not be accompanied with changes in the composition and structure of the crystal. Since impurities will not be mixed in, problems such as degradation of the crystal or the degradation in the resistance to the optical damage will not occur. A high nonlinearity and resistance to optical damage can be realized, and thus an optical waveguide device with high efficiency and high output can be provided. However, since there is no change in the crystal composition, changes in refractive index will not occur and thus it cannot be used for a DBR grating.
As there is no change in the crystal composition in a domain-inverted structure, optical changes will not appear and thus the domain-inverted structure cannot be used as a refractive index grating structure just like the prior art. That is, when there is no difference in the crystal structures in the domain-inverted portion and the non-inverted portion, no optical difference will occur. Therefore, a refractive grating structure will not be formed.